Skid steer or tracked vehicles such as mini hydraulic excavators (MHE) commonly have an open loop hydraulic transmission for propelling and steering the vehicle. One simplified example of an open loop system may have a fluid reservoir for a fluid such as hydraulic oil, of which a pump can draw a volume of fluid. The pump pressurizes the fluid and sends this to at least one of two motors (one for each track) which is coupled to a track such that the rotating motor will turn the track thereby moving the vehicle. Once the fluid has passed through the motor the fluid is returned to the reservoir. For comparison, in a closed loop system the return flow from the motor would return to the pump instead of the reservoir. Closed loop systems tend to provide superior vehicle control, but generally require more sophisticated pumps, motors and control software resulting in higher levels of complexity and expense.
In a closed loop arrangement the return flow can flow no faster than what the pump can take in. For example, on an inclined terrain the vehicle will tend to roll downwards whereby the track is inclined to drive the motor instead of the motor driving the track. However, the motor cannot displace more fluid than the pump can take in, therefore the motor is prevented from speeding up and the operator remains in control of vehicle speed. In an open loop system without any additional controls in place, the motor would be able to freely dispose of the fluid into the tank leading to potentially uncontrolled vehicle behavior such as a run away condition of the machine. To prevent this situation, over center lock valves may be fitted in the motor return line. As the motor is bi-directional, an over center lock valve is fitted in both supply/return lines to each motor.
Steering of vehicles with open loop steering arrangements is achieved by providing the two motors with different fluid quantities or opposite flows, thereby causing the tracks to have different speeds and/or directions. One problem associated with open loop systems is the repeated switching of the over center lock valves in response to varying flows and pressures created during the steering process. Operator control may not always be smooth and the system is affected by changing torque and power requirements during a steering maneuver. This causes the over center valve to open and close repeatedly making the steering maneuver jerky and uncontrolled. This may lead to problems with modern machinery where high travel speeds are demanded to reduce travel times when moving between sites. To avoid those problems, an operator may have to slow down the vehicle or come to a complete standstill before engaging a turn. To increase smoothness during a turn the machine may be equipped with cross line relief valves between the motor inlet and outlet ports as this enables a continued rotation of the braked motor as fluid is moved across the relief valve during turning hence softening the turn to a degree. As the range relief valve settings are limited due to system relief valve settings, cross line relief is only a partial alleviation of some of the problems.
The following disclosure is directed to one or more improvements in the existing technology.